Multi-mode interfrence optical waveguide device

ABSTRACT

A multi-mode interference (MMI) device, comprising a hollow core multi-mode waveguide optically coupled to at least one hollow core input waveguide, is described in which the internal surfaces of the hollow core waveguides carry a reflective coating. The coating may be a low refractive index material at the wavelength of operation, such as a metal, or a multiple layer dielectric stack. Resonators and optical amplifiers using such (MMI) devices are also described.

This invention relates to multi-mode interference (MMI) opticalwaveguide devices.

U.S. Pat. No. 5,410,625 describes a multi-mode interference (MMI) devicefor beam splitting and recombining. The device comprises a firstcoupling waveguide and two or more second coupling waveguides that areconnected to a central multi-mode waveguide region. The couplingwaveguides operate only in fundamental mode, and the physicalcharacteristics of the coupling and multi-mode waveguide regions areselected such that modal dispersion within the central multi-modewaveguide region provides for a single beam of light input in to thefirst coupling waveguide to be split into the two or more secondcoupling waveguides. The device may also be operated in reverse as abeam combiner.

Variations and improvements to the basic MMI devices of U.S. Pat. No.5,410,625 are also known. U.S. Pat. No. 5,379,354 describes howvariation of input guide location can be used to obtain a multi-way beamsplitter that provides division of the input radiation into outputsbeams having differing intensities. Use of MMI devices to form lasercavities has also been demonstrated in U.S. Pat. No. 5,675,603. Variouscombinations of MMI splitter and recombiner devices have also been usedto provide an optical routing capability; for example, see U.S. Pat. No.5,428,698.

Solid core MMI waveguide devices are known in which the coupling andmulti-mode waveguides are formed from solid ridges of semiconductormaterial, such as Gallium Arsenide (GaAs), that are upstanding from asubstrate. Solid core MMI waveguide devices are typically fabricatedfrom layers of GaAs. A disadvantage of solid core materials is thelimited total power density which they can transmit before damage to thesolid core material occurs.

MMI devices are also known in which the coupling and multi-modewaveguides are formed as hollow cavities (i.e. air cavities) withinsubstrates of solid dielectric material, such as alumina. The dielectricsubstrate material is selected to have a refractive index less than theair core at the particular wavelength of operation of the device. Hollowcore dielectric devices are typically fabricated by a precisionengineering (e.g. milling) process and are typically physically largerin size than their solid core counterparts. Precise control over thedimensions of such devices, which is important for obtaining optimumperformance, can also prove challenging.

It is an object of this invention to provide an alternative MMI opticalwaveguide device.

According to a first aspect of the present invention, a multi-modeinterference (MMI) device comprises a hollow core multi-mode waveguideregion optically coupled to at least one hollow core input waveguide,characterised in that the internal surfaces of the hollow corewaveguides carry a reflective coating.

Hollow core MMI devices of the present invention may be operated as beamcombiners, beam splitters, multi-way beam intensity dividers etc.

An advantage of the present invention is that the hollow core waveguidestructures (i.e. the substrate that defines the hollow core waveguidestructures) on which the reflective coating is located may be formedfrom any material. This is an advantage over prior art hollow core MMIdevices which are fabricated from specific materials (such as alumina)to ensure optical losses were minimised. The present invention thuspermits the waveguides to be fabricated using a variety of materials andprocesses that was not previously thought possible by those skilled inthe art. In particular, this invention provides the opportunity tofabricate physically small hollow core waveguide devices using highprecision micro-fabrication techniques; the restrictions on minimumhollow core device size that were imposed by the use of traditionalprecision engineering techniques have thus been overcome.

It should also be noted that the hollow core waveguides may be producedin a variety of ways. The waveguides may be formed in unitary pieces ofmaterial, they may be formed from two separate pieces of material (suchas a base and a lid) or they may be formed from a plurality of differentpieces of material (e.g. separate sections of material that, whenlocated together, define the required fundamental mode and multi-modewaveguide regions).

The hollow core waveguides of the present invention allow deviceoperation with high levels of optical power. This is an advantage overprior art solid core waveguides, in which the maximum optical powerdensity is limited by the physical properties of the material formingthe solid core.

Advantageously, the reflective coating comprises a layer of materialhaving a refractive index less than that of the waveguide core withinthe operating wavelength band. The layer of material having a refractiveindex lower than the hollow waveguide core produces total internalreflection (TIR) of light within the MMI device thereby providing ahollow core device having low associated levels of optical loss.

It should be noted that when hollow core optical waveguide structuresare produced, the hollow core is likely to fill with air. Herein therefractive index of the core is thus assumed to be that of air atatmospheric pressure and temperature (i.e. n≈1). However, this should beseen in no way as limiting the scope of this invention. The hollow coremay contain any fluid (for example an inert gas such as nitrogen) or bea vacuum. The term hollow core simply means a core which is absent anysolid material. Also, the term total internal reflection (TIR) shall betaken herein to include attenuated total internal reflection (ATIR).

In a further embodiment, the layer of low refractive index materialcarried on the internal surface of the hollow core waveguides is ametal; for example gold, silver or copper.

The properties of gold, silver and copper therefore make these metalsparticularly suited to inclusion in MMI devices for operation in thetelecommunications wavelength band (i.e. for use with wavelengthscentred around 1.55 μm).

Metals will exhibit a suitably low refractive index over a wavelengthrange that is governed by the physical properties of the metal; standardtext books such as “the handbook of optical constants” by E. D. Palik,Academic Press, London, 1998, provide accurate data on the wavelengthdependent refractive indices of various materials. In particular, goldhas a refractive index less than that of air for wavelengths within therange of around 1400 nm to 1600 nm. Copper exhibits a refractive indexless than unity over the wavelength range of 560 nm to 2200 nm, whilstsilver has similar refractive index properties over a wavelength rangeof 320 nm to 2480 nm.

The layer of metal may be deposited using a variety of techniques knownto those skilled in the art. These techniques include sputtering,evaporation, chemical vapour deposition (CVD) and (electro orelecto-less) plating. CVD and plating techniques allow the metal layersto be deposited without any direction dependent thickness variations.Plating techniques also permit batch processing to be undertaken.

A skilled person would recognise that adhesion layers and/or barrierdiffusion layers could be deposited on the hollow core waveguide priorto depositing the layer of metal. For example, a layer of chrome ortitanium could be provided as an adhesion layer prior to the depositionof gold. A diffusion barrier layer, such as platinum, may also bedeposited on the adhesion layer prior to gold deposition. Alternatively,a combined adhesion and diffusion barrier layer (such as titaniumnitride or titanium tungsten alloy or an insulator such as siliconoxide) could be used.

Conveniently, the reflective coating may also comprise one or morelayers of dielectric material. The dielectric material may be depositedby CVD or sputtering. Alternatively, a dielectric layer could be formedby chemical reaction with a deposited metal layer. A deposited layer ofsilver could be chemically reacted with a halide to produce a thinsurface layer of silver halide. For example, a silver iodide (AgI)coating could be formed on the surface of silver by exposing it to I₂ inthe form of a potassium iodide (KI) solution

In other words the reflective coating may be provided by anall-dielectric, or a metal-dielectric, stack. A person skilled in theart would recognise that the optical thickness of the dielectriclayer(s) gives the required interference effects and thus determines thereflective properties of the coating. The reflective properties of thecoating may also be dependent, to some extent, on the properties of thematerial in which the hollow core waveguides are formed.

The device may advantageously be configured to operate across thewavelength range 0.1 μm and 20.0 μm, and more preferably in theinfra-red bands of 3-5 μm or 10-14 μm. Advantageously, the deviceoperates with radiation between 1.4 μm and 1.6 μm in wavelength.

Conveniently, the at least one hollow core input waveguide is afundamental mode waveguide. Alternatively, the at least one hollow coreinput waveguide is a multi-mode waveguide. As described in more detailbelow, a fundamental mode or multi-mode waveguide may be used to coupleradiation into, or out of, the hollow core multi-mode waveguide region.

Preferably, the at least one hollow core input waveguide comprises ahollow core optical fibre. In other words, a hollow core optical fibremay be arranged to directly couple radiation into the multi-modewaveguide region.

Advantageously, the device additionally comprises an optical fibre thatis directly optically coupled to the hollow core multi-mode waveguideregion. The optical fibre may comprise a hollow or solid core. It wouldalso be apparent to the skilled person that such a solid core opticalfibre could be used in place of the at least one hollow core inputwaveguide of the present invention.

Conveniently, the hollow core multi-mode waveguide region has asubstantially rectangular cross-section. As described below, this canprovide an MMI beam splitter or recombiner. It is preferred for thedimensions (i.e. width, length and depth) of such a hollow coremulti-mode waveguide region to be selected to provide re-imaging (i.e.to produce one or more images of the input beam) of the optical inputfield carried by said at least one hollow core input waveguide.

Conveniently, opposite surfaces forming the rectangular internalcross-section of the hollow core multi-mode waveguide region havesubstantially equal effective refractive indices and adjacent surfacesforming the rectangular internal cross-section hollow core multi-modewaveguide region have different effective refractive indices. In thismanner, the device can be arranged to have reduced optical losses whenguiding light of a known linear polarisation.

In some embodiments, the hollow core multi-mode waveguide region mayhave a substantially circular cross-section and the diameter and lengthof the hollow core multi-mode waveguide region are selected to providere-imaging of the optical input field carried by said at least onehollow core input waveguide. It should be noted that beam splitting isnot possible with such a circular multi-mode region, only re-imagingeffects are observed.

In a further embodiment, the layer of material carried on the internalsurface of the hollow core waveguides is Silicon Carbide. As describedabove, the additional layer of low refractive index material can beselected to provide efficient MMI operation at any required wavelength.Silcon Carbide has a refractive index of 0.06 at 10.6 μm, making suchmaterial particularly suited for inclusion in MMI devices operating atsuch a wavelength.

Conveniently, the hollow core waveguides are formed in semiconductormaterial; for example silicon or III-V semiconductor materials such asGaAs, InGaAs, AlGaAs or InSb. The semiconductor material may be providedin wafer form. Advantageously, the hollow core waveguides are formedusing semiconductor micro-fabrication techniques. Preferably, suchmicro-fabrication techniques provide fundamental mode waveguides havingcross-sections of less than 3 mm, or more preferably less than 1 mm.

A person skilled in the art would recognise that micro-fabricationtechniques typically involve a lithography step, followed by an etchstep to define the pattern in the substrate material or a layer thereon.The lithography step may comprise photolithography, x-ray lithography ore-beam lithography. The etch step may be performed using ion beammilling, a chemical etch, a dry plasma etch or a deep dry (also termeddeep silicon) etch. Preferably, Deep Reactive Ion Etching (DRIE)techniques are used.

Waveguides formed using micro-fabrication techniques of this typeprovide hollow core waveguides that are significantly smaller in sizethan prior art hollow dielectric waveguides. Micro-fabricationtechniques of this type are also compatible with various layerdeposition techniques such as sputtering, electroplating, CVD or otherreactive chemistry based techniques.

In a further embodiment, the hollow core waveguides are formed fromplastic or a polymer. For example, the hollow core waveguides could beformed using a lithographic process on a “spin-on” polymer coating (e.g.SU8 available from Microchem. Corporation)

Plastic waveguide devices may be fabricated by techniques including hotembossing or injection moulding. The technique involves forming amaster. The master may be formed in semiconductor material, such assilicon, using a deep dry etch.

Alternatively, the master may be formed by electro deposition of layersusing the LIGA or UV LIGA technique. Once the master is formed, thehollow core waveguides may be formed in a plastic substrate by stamping(i.e. pressing) or hot stamping. The hollow plastic waveguides thusformed may then be coated with a reflective coating.

In a further embodiment, the hollow core waveguides are formed fromglass; such as quartz, silica etc.

Conveniently, the hollow core of the device comprises a liquid or a gassuch as air.

A gaseous optical gain medium may also be advantageously used to provideamplification of light within the hollow core waveguides. In particular,the use of such a gaseous gain medium in the hollow core multi-moderegion permits a high degree of amplification. For example the gaseousgain medium could be a gas discharge formed in a mixture of CO₂, N₂ andHe. This would provide amplification for 10.6 μm radiation.

According to a second aspect of the invention, an optical amplifiercomprises a 1-to-N way beam splitter, a multiple element opticalamplifier, and a beam recombiner connected in optical series, theoptical amplifier acting on at least one of the outputs of the 1-to-Nway beam splitter, wherein at least one of the 1-to-N way beam splitterand beam recombiner comprise a hollow core multi-mode interferencedevice according to the first aspect of the invention.

In other words, an optical amplifier incorporates an MMI deviceaccording to the first aspect of the invention. The use of such an MMIdevice, permits the amplifier to provide large amounts of optical power.This is advantageous over prior art amplifiers fabricated from solidcore waveguides, in which the maximum optical power density is limitedby the physical properties of the material forming the core.

A high order splitting-amplification-recombination is thus possible,thereby allowing the production of high intensity output beams notpreviously attainable.

Conveniently, the 1-to-N beam splitter and the beam recombiner bothcomprise hollow core multi-mode interference devices according to thefirst aspect of the invention. Alternatively, the 1-to-N beam splittercomprises a solid core MMI splitter device.

In a further embodiment, the optical amplifier further comprises phaseoffset means to adjust the relative phases of the amplified beams priorto beam recombination in the beam recombiner. The phase offset means,which may comprise GaAs modulators or deformable mirrors etc, allows therelative phases of the beams entering the recombiner to be controlled.Ensuring that the beams entering the recombiner device have appropriatephase offsets will increase the efficiency of the recombination processand will allow the recombination region to be shorter in length(especially in high order splitting/recombining devices).

According to a third aspect of the present invention, a resonatorcomprises a partial reflector, a splitter/recombiner means, amulti-element optical amplifier, and a reflector, the partial reflector,splitter/recombiner means, multi-element optical amplifier and reflectorbeing arranged such that the splitter/recombiner means splits a singlebeam into N beams where N is greater than or equal to 2, each of the Nbeams are amplified by the multi-element optical amplifier, reflected bythe reflector and redirected to pass back through the multi-elementamplifier, the N beams then being recombined by the splitter/recombinermeans to form a single beam, a portion of that single beam exiting theresonator through the partial reflector, wherein the splitter/recombinermeans is a hollow core multi-mode interference device according to thefirst aspect of the present invention.

The resonator is effectively a amplifier folded back on itself, andprovides the capability for high optical power operation with low levelsof optical loss.

The invention will now be described, by way of example only, withreference to the accompanying drawings in which;

FIG. 1 illustrates a prior art hollow core MMI splitter device and thetransverse electric field profile of such a device;

FIG. 2 illustrates a prior art solid core MMI splitter device;

FIG. 3 shows an MMI waveguide device according to the present invention;

FIG. 4 shows a comparison of experimental data recorded from an MMIdevice of the present invention and an uncoated hollow core MMI device;

FIG. 5 shows an amplifier and resonator optical circuit incorporatingtwo-way MMI splitter/recombiner devices according to the presentinvention;

FIG. 6 shows an amplifier and resonator optical circuit incorporatingfour-way MMI splitter/recombiner devices according to the presentinvention;

FIG. 7 shows an alternative configuration for an amplifier incorporatingMMI splitter/recombiners devices according to the present invention; and

FIG. 8 shows a hybrid optical amplifier circuit incorporating solid coreMMI splitter devices and a MMI recombiner according to the presentinvention.

Referring to FIG. 1(a), a perspective view of a prior art two way hollowcore waveguide MMI beamsplitter 22 is shown. The MMI beamsplitter 22comprises a substrate layer 24, a waveguide layer 26 and a cover layer28. The waveguide layer 26 defines a hollow core waveguide structurehaving an input waveguide 30, a multi-mode waveguide region 32 and twooutput waveguides 34 and 36.

The hollow core multimode waveguide region 32 is rectangular; being oflength 1 and width W. The input waveguide 30 is ported centrally to, andthe output waveguides 34 and 36 are located with the port centres spacedapart across, the multi-mode waveguide region 32. The input waveguide 30and the output waveguides 34 and 36 are arranged so to support onlyfundamental mode propagation.

In operation, the fundamental mode supported by the input waveguide 30is ported into the central multi-mode waveguide region 32. The length(l) and width (w) of the multi-mode waveguide region 32 are selected sothat multi-mode interference along its length produces a equal divisionof the input beam intensity which is coupled into the output waveguides34 and 36. In this manner, a single input beam of radiation can be splitinto two output beams. It is also possible to operate the device inreverse to combine two beams.

Referring to FIG. 1(b), the basic principle underlying the multi-modeinterference that provides beam splitting is shown. FIG. 1(b)illustrates transverse intensity profiles for electromagnetic radiationof wavelength λ at thirteen equally spaced positions along a rectangularmulti-mode waveguide region of length L and width W where L=W²/λ. It isassumed that the incident radiation input (i.e. the mode represented bythe first transverse intensity curve 56) is a fundamental mode.

It can be seen from FIG. 1(b) that a device of length less than L, maybe used to perform a beam splitting function. In the case of a two waybeam splitter of the type described in FIG. 1(a), a device of length L/2(=1) is required. Similarly, three way or four way splitters can beprovided if they are of length L/3 and L/4 respectively. In other words,a N-way split can be obtained with a device of length L_(N)=W²/Nλ. Amore complete explanation of the operation and design of MMI splitterdevices is given in U.S. Pat. No. 5,410,625.

Those skilled in the art have, to date, constructed hollow corewaveguide structures using dielectric substrate materials having arefractive index less than air (i.e. n<1) at the particular wavelengthof operation. In particular, alumina substrates have been used becausethey have a refractive index, for light of 10.5 μm in wavelength, lessthan air. This ensures that light propagating through the hollow corewill undergo TIR at the interface between air and the substrate.

A disadvantage of using alumina, and other dielectric materials, is thatsuch materials are typically shaped to form hollow core MMI devicesusing precision engineering (e.g. milling or sawing) techniques. Thesefabrication techniques limit the minimum size of hollow core device thatcan be created whilst maintaining the tolerances required for MMI deviceoperation. For example, typical milling techniques allow structures tobe defined with a tolerance of not less than 50 μm in waveguidestypically not less than 1 mm in width.

Alternative materials that can be used to fabricate hollow core devicesof a smaller physical size do not provide sufficient levels ofreflection at the air-substrate interface and significant optical lossesarising from the Fresnel reflection of light at the interface betweenthe hollow core and the surrounding material are introduced. The opticalloss associated with Fresnel reflection in MMI devices, which isexacerbated in smaller size devices, has led those skilled in the art todiscount the use of hollow core substrates to produce small size MMIdevices. Effort has thus been expended, in the drive for smaller devicesizes, on producing solid core MMI waveguide devices.

Referring to FIG. 2, a four-way solid core MMI waveguide structure 60 isshown. The solid core MMI waveguide structure 60 consists of asemi-insulating GaAs substrate layer 62, a lower cladding layer 64, aGaAs core layer 66 and an upper cladding/capping layer 68.

An input waveguide 70 is ported centrally on to a multi-mode waveguideregion 80 of width W and length l′ and four output waveguides (72,74,76and 78) are also provided. The input and output waveguides are arrangedso as to only support fundamental mode propagation. The length (l′) ofthe multi-mode waveguide region 80 is L/4 (where L=W²/λ.) to provide afour way split.

The refractive index of the GaAs core is around 3.5, whilst thesurrounding air has a refractive index of around 1. Total internalreflection (TIR) is thus obtained at the interface between the GaAsmaterial and the surrounding air. The TIR that occurs at the interfacebetween the GaAs and air provides a surface reflectivity substantiallygreater than that found in hollow core device. The overall opticalefficiency of the solid core devices of this type is thereforesignificantly greater than hollow core equivalents.

A disadvantage of solid core MMI splitter devices is that only a limitedamount of optical power can be propagated in the solid core beforedamage to the material forming the core occurs. The power handlingcapabilities of solid core MMI devices are therefore limited; thisplaces a limitation on the use of such devices in high powerapplications such as optical amplifiers etc.

Referring to FIG. 3, a perspective view of a two way hollow corewaveguide MMI beamsplitter 90 according to the present invention isshown; similar elements to those described in previous figures are givenlike reference numerals.

The MMI beamsplitter 90 comprises a substrate 88 and a substrate lid 86.The substrate 88 and the substrate lid 86 define a hollow core waveguidestructure having an input waveguide 30, a multi-mode waveguide region 32and two output waveguides 34 and 36. A layer of gold 92 (indicated bythe hatch markings in FIG. 3) is provided on the inner surfaces of thesubstrate 88 and the substrate lid 86 that define the hollow corewaveguide structure. The gold layer 92 should be sufficiently thick toensure ATIR takes place. A person skilled in the art would recognisethat an adhesion promoting layer and/or a diffusion barrier layer (notshown) may also be provided in-between the layer of gold 92 and thesubstrate.

Apart from any alteration to the length and width of the cavity causedby the addition of the layer of gold metal, the layer of gold 92 doesnot affect the design of the MMI device. The input waveguide 30, themulti-mode waveguide region 32 and the two output waveguides 34 and 36are designed using the same criteria used for prior art hollow core MMIdevices of the type described with reference to FIG. 1.

The presence of the layer of gold 92 provides ATIR within the hollowcore device for light with a wavelength within the telecommunicationswavelength band (i.e. for wavelengths around 1.55 μm). At thesetelecommunication wavelengths, gold has the required refractive indexproperties of n<1 and low absorption levels.

Although a gold layer 92 is described above, a person skilled in the artwould recognise that any material having a refractive index less thanair (or whatever is contained within the cavity) at the wavelengths atwhich the waveguide is to be operated could be deposited on the surfacesdefining the hollow core waveguide. The refractive indices of differentmaterials can be found in various publications, such as “the handbook ofoptical constants” by E. D. Palik, Academic Press, London, 1998. Metalstypically have a refractive index less than air over a given wavelengthrange; the particular wavelength range depending on the physicalproperties of the metal. The low refractive index of metals at aparticular wavelength is generally accompanied by an absorption maximaacross a similar wavelength range. Hence, a material should preferablybe selected with a refractive index less than air and also with lowabsorption at the wavelength or wavelengths of device operation.

A skilled person would recognise that, instead of using a single lowrefractive index layer, multiple layer reflectors could be provided. Forexample, multiple layer dielectric stacks or metal-dielectrics stackscould be coated on the substrate 88 and/or the substrate lid 86.

A suitable material for the substrate 88 and the substrate lid 86 issilicon; silicon can be etched to a very high degree of accuracy usingmicro-fabrication techniques of the type known to those skilled in theart. Any material that can be formed in the required physical geometrycould be used to produce the MMI device. However, the use ofmicro-fabricated semiconductors is particularly advantageous as isallows devices to be made that are significantly smaller in size thanprecision engineered alternatives; micro-fabrication processes canprovide sub 1 μm accuracy. Micro-fabrication also allows multiplestructures to be formed in parallel on the substrate, this is unlikeprecision engineering techniques in which waveguide structures areformed serially by moving a cutting tool across the surface of thesubstrate.

Ideally, the substrate 88 and the substrate lid 86 should be fabricatedfrom a material suitable for coating with a layer of the low refractiveindex material. A person skilled in the art would appreciate how thedeposition of layers of gold onto silicon, using metal depositiontechniques such as sputtering, evaporation, CVD, or plating, can beachieved. A skilled person would also appreciate that the lid could bebonded to the substrate via techniques such as a gold-silicon eutecticbonding or an intermediate layer.

Referring to FIG. 4, experimental data demonstrating the transmissionproperties of two-way MMI devices of the present invention are shown.

Two-way hollow core waveguide MMI beamsplitters having a multi-moderegion width (W) of 250 μm and fundamental mode waveguide widths of 50μm were constructed. The devices were fabricated using variousmulti-mode region lengths (l), and with and without a coating of coppermetal applied to the internal surface of the hollow core waveguidestructure using a nickel adhesion layer.

The first curve 100 shows the total transmission of light through twoway hollow core MMI splitters, whilst the second curve 102 shows thetransmission of light through two way hollow core MMI splitters with acoating of copper metal applied to their internal surfaces. It isapparent from the experimental data that the application of a layer ofcopper material to the internal surface of the hollow core beamsplitterwill almost double the transmission efficiency of the device. This makesthe device a practical alternative to solid core devices.

Referring to FIG. 5, several applications are schematically illustratedin which hollow core two-way splitter/combiner MMI devices according tothe present invention can be employed.

FIG. 5 a shows an amplifier 110. The amplifier 110 comprises a splitterstage 112, a diode array amplifier 114 and a combiner stage 116.

The splitter stage 112 comprises a first two-way MMI splitter 118 andtwo secondary two-way MMI splitters 120 and 122. Each of the two-way MMIsplitters 118, 120 and 122 comprise a single input waveguide 124, twooutput waveguides 126 and a central multi-mode region 128. The inputs ofthe secondary two-way MMI splitters are connected to the outputs of thefirst two-way MMI splitter 118.

The diode array amplifier 114 comprises 4 separate amplificationelements (130 a,b,c,d) that are optically connected between the fouroutputs of the splitter stage 112 and the four inputs of the combinerstage 116. Laser diode arrays of this type are well known to thoseskilled in the art.

The combiner stage 116 comprises a pair of two-way MMI combiners 132 and134, and a second MMI combiner 136. Each of the two-way MMI combiners132, 134 and 136 comprise a pair of input waveguides 138, a singleoutput waveguide 140 and a central multi-mode region 128. The outputs ofthe pair of two-way MMI combiners 132 and 134 are connected to theinputs of the second MMI combiner 136.

In operation, the splitter stage 112 divides an incident light beam 142into four beams; each of equal intensity. The four element diode arrayamplifier 114 then amplifies each of the four beams, before they arerecombined in the combiner stage 116. An amplified resultant output beam144 thus results.

Referring to FIG. 5 b, a resonator structure 150 is shown. The resonator150 is effectively an amplifier of the type described with reference toFIG. 5 a folded back on itself.

The resonator 150 comprises a single MMI stage 152 that has a firsttwo-way MMI splitter/combiner 154 and two secondary two-way MMIsplitters/combiners 156 and 158. Each MMI splitter combiner 154, 156 and158 has a first waveguide 160, two second waveguides 162 and amulti-mode region 128. The two second waveguides of the first two-wayMMI splitter/combiner 154 are optically connected to the firstwaveguides of the two secondary two-way MMI splitters/combiners 156 and158. The resonator also comprises a fully reflecting mirror 164 and apartially reflecting mirror 166 and a four element diode array amplifier114.

In operation the MMI stage 152 performs both a splitting and combiningfunction, and the resonator provides light amplification. The partiallyreflecting mirror 166 allows a proportion of the light to be extractedas an output beam 168.

Although the amplifier and resonators described with reference to FIG. 5could be fabricated using known MMI splitters/combiners, it is preferredto use MMI device of the type described with reference to FIGS. 3 and 4.The use of hollow core waveguides without the low refractive indexcoating would increase the cumulative losses associated with each MMIsplitter/recombiner in the system, thereby reducing the lightamplification of the diode array amplifier 114. Also, if prior art solidcore (e.g. GaAs) MMI splitters/combiners were used the power densitiesassociated with the recombination stage could cause significantdegradation of the solid core material. The present invention thusprovides optical amplifiers and resonators having high power handlingcapabilities.

Referring to FIG. 6, a further example of how hollow core MMI devicesaccording to the present invention can be advantageously employed isdescribed.

FIG. 6 a shows a 1-to-4-to-1-way amplifier. The amplifier comprises afirst four way MMI splitter 180 having an first waveguide 182, amulti-mode region 184 and four second waveguides 186. A four elementdiode array amplifier 114 is also provided, along with a second four wayMMI recombiner 190.

In operation, incident light 192 is coupled into the first waveguide 182of the first four way MMI splitter 180. The first four way MMI splitter180 equally splits the light between its four second waveguides 186 andpasses the light to the four element diode array amplifier 114.

Light emerging from each of the four second waveguides of the first MMIsplitter 180 is amplified by each element of the four element diodearray amplifier 114, before entering the second waveguides of the secondfour way MMI recombiner 190. The second four way MMI recombiner 190 thenrecombines the four amplified light beams to form a single, andamplified, output beam 194.

However, and unlike the two-way splitters described above, the phases ofthe four light beams entering the MMI recombiner 190 need to beconsidered. Such phase considerations are only required when the MMIdevices are designed to split and recombine three or more beams (i.e.when N≧3) using the shortest possible multi-mode region lengths.

As described above with reference to FIG. 2, an N-way split can beobtained with the shortest multi-mode region length when the MMI devicehas a multi-mode region of width W and length L_(N)=W²/Nλ. It shouldalso be noted that the wavelength λ is the wavelength of light in themultimode region (i.e. the free-space wavelength of light multiplied bythe refractive index of the core material).

In terms of the pitch (p) of the axes of the array elements, themulti-mode guide length (l) for an N-way split can be written as:$\begin{matrix}{l_{N} = \frac{({Np})^{2}}{\lambda}} & (1)\end{matrix}$where p is the pitch of the second waveguides (e.g. the pitch of thefour waveguides 186 of the MMI splitter 180) and p=W/N. It can be seenfrom equation (1) that the length of the multi-mode guide (l) regionscales linearly with the order of the split (i.e. N) for a fixed pitch.

The result of the symmetric splitting process in multi-mode waveguidesobeying the geometric design rules described above, is that Nfundamental mode fields are produced that have equal amplitude. Thephase of the resulting fields are not however equal, and are governed bythe relationship; $\begin{matrix}{\phi_{n} = {\left\{ {\frac{1}{2N} + \frac{N + 1}{4} + {\frac{n}{N}\left( {n - N - 1} \right)}} \right\}\pi}} & (2)\end{matrix}$

In the case of a four way splitter (i.e. N=4), the relative phases ofthe 4 output fields are therefore 3/8π, 1/8π, 1/8π and 3/8πrespectively.

In order to efficiently recombine 4 beams using a four way recombiner,the phases of the fields entering the multi-mode waveguide region haveto take on values that are the exact phase conjugate of those producedby the splitting process. In other words the phases of the four inputfields to the multimode region of MM recombiner 190 must be −3/8π, 1/8π,1/8π and −3/8πrespectively for efficient recombination.

Following the above, it is advantageous to introduce phase offsetsbetween the MMI splitter 180 and the MMI recombiner 190 which allowthese phase conditions to be satisfied. In general terms the phaseoffsets required between a 1-to-N way splitter and a N-to-1 wayrecombiner are given by; $\begin{matrix}{\phi_{n} = {{- 2}\left\{ {\frac{1}{2N} + \frac{N + 1}{4} + {\frac{n}{N}\left( {n - N - 1} \right)}} \right\}\pi}} & (3)\end{matrix}$

To establish the required phase offsets in the 1-to-4-to-1-way amplifierof FIG. 6(a), phase off-set means 196 are provided on each of the foursecond waveguides 186 of the MMI recombiner 190. The phase off-set means196 comprises modifications to the physical lengths of the guidesfeeding the MMI recombiner 190.

Numerous alternative techniques for producing the required phaseoff-sets are also known to those skilled in the art. For example, thecurrent in each element of the diode array amplifier 114 could bealtered. Alternatively, the optical path length within the diode arrayamplifier 114 could be altered or the effective refractive index of asection of the waveguide or diode array amplifier 114 could be modifiedto provide the necessary phase shift.

FIG. 6 b illustrates a resonator that consists of a single four way MMIsplitter/recombiner 200 and a four element diode array amplifier 114.The four way MMI splitter/recombiner 200 has a multi-mode region 184, afirst waveguide 182 and four second waveguides 186. Phase off-set means204 are provided on each of the four second waveguides 186. Theresonator also comprises a fully reflecting mirror 164 and a partiallyreflecting mirror 166.

The resonator is effectively an amplifier folded back on itself, withthe result that a double pass through the four way MMIsplitter/recombiner 200 results in light amplification. As light passesthrough the phase off-set means 204 twice during each double passthrough the device, the phase off-sets provided by the phase off-setmeans 204 is half that given in equation (3) above for a amplifierdevice. The partially reflecting mirror 166 allows a proportion of thelight to be extracted as an output beam 202.

Again, hollow core MMI splitters/recombiners of the type described withreference to FIGS. 3 and 4 are advantageous as they provide the abilityto handle high optical powers with low levels of attenuation.

Referring to FIG. 7, an alternative embodiment of the amplifierdescribed with reference to FIG. 6(a) is provided. The amplifiercomprises a 7-way MMI splitter 210, a 7 element diode array chip 212 anda 7-way MMI recombiner 214. The 7-way MMI splitter 210 has an inputwaveguide 216 and a multi-mode region 218. The 7-way MMI combiner 214has an output waveguide 220 and a multi-mode region 218.

The multi-mode regions 218 of both the MMI splitter 210 and the MMIcombiner 214 are directly optically coupled to either side of the 7element diode array chip 212. The dimensions of the multi-mode region218 of the MMI splitter 210 are such that a incident fundamental modeentering that region from the input waveguide 216 is split into 7 beamsof equal intensity at the interface 222 with the diode array chip 212.The 7 beams are then amplified by the diode array chip 212, beforeentering the MMI recombiner 214 when they are recombined to form asingle beam that exits the device through the output waveguide 220. Inthis device, any necessary phase offsets are provided in the diode arraychip region.

In common with the amplifier and resonator devices described withreference to FIG. 6, the integrated amplifier device of FIG. 7advantageously comprises an MMI splitter 210 and/or an MMI recombiner214 of the type described with reference to FIGS. 3 and 4

Referring to FIG. 8, a hybrid amplifier is shown. The hybrid amplifiercomprises a solid core MMI splitter 230, six phase shifting means 232, atapered diode amplifier array 234 and a hollow core MMI combiner 236.

The solid core MMI splitter 230 is fabricated from GaAs and has a singleinput waveguide 238, a multi-mode region 240 and six output waveguides242. The width (w₁) and length (l₁) of the multi-mode region 240 isselected such that an input beam 241 coupled into the input waveguide238 is split into the 6 output waveguides 242.

Each of the 6 output waveguides 242 fan out to a phase shifting means232. The phase shifting means 232 comprise electro-optic modulators,again fabricated from GaAs, that impose a phase shift to the opticalbeam on application of a suitable voltage. The phase shifts applied toeach beam to ensure efficient recombination is governed by equation (3)above. The phase shifting means 232 also compensates for the phaseerrors introduced by the fan out process itself. Phase errors that areintroduced during the manufacturing process, such as cleave error,inconsistencies in the waveguide properties, can also be compensated forby the phase shifting means 232.

It should be noted that as well as needing to achieve appropriate phaseoff-sets for efficient recombination, the beams must also be of equalamplitude. Equal amplitude correction could be achieved in a variety ofways that are known to those skilled in the art. For example, aMach-Zehnder variable attenuator (not shown) could be placed on eachoutput waveguides 242 before the phase shifting means 232.

The phase shifted beams which exit the phase shifting means 232 arecoupled in to a tapered diode amplifier array 234, which individuallyamplifies each of the 6 beams. A tapered amplifier suitable for thistask is described in P Wilson et al, Electronics letters, 7 Jan. 1999,Vol.35 No.1.

Once amplified, the 6 optical beams are coupled directly into themulti-mode region 244 of the hollow core MMI recombiner 236. To ensurereflections are mininised at the interface between the solid elements ofthe tapered diode amplifier array 234 and the hollow core multi-moderegion 244, an anti-reflection coatings 246 is provided.

The width (w₂) and length (l₂) of the multi-mode region 244 are selectedsuch that the six amplified beams entering that region are recombinedand the amplified output beam 247 exits the device through the outputwaveguide 248. It should be noted that the dimensions of the solid coreMMI splitter 230 and the hollow core MMI recombiner 236 are differentbecause of the difference in refractive index of GaAs and air(approximately 3.5 compared to 1.0 respectively); this makes the solidcore MMI splitter 230 physically smaller in size than the hollow coreMMI recombiner 236.

Although a 1-to-6-to-1 amplifier is described above, a person skilled inthe art would recognise that a much higher degree of splitting andrecombining is possible. As the amount of power that requires opticalcombination increases, the power handling capabilities of the recombinermust increase accordingly.

In a hybrid amplifier of the type described in FIG. 8, solid core MMIdevices are used to split the incident beam of radiation as such devicesare typically more compact than hollow core equivalents and will provideefficient beam splitting of a low power incident beam. However, whenrecombining the amplified signals solid core devices would be unable tohandle the increased optical power without damage to the solid corematerial occurring. The use of an MMI recombiner of the type describedwith reference to FIGS. 3 and 4 allows efficient recombination of thehigh intensity beams without the possibility of core damage to therecombiner device.

The MMI devices described above provides a split in one dimension (e.g.horizontal). It is however possible to also provide a split in a second(e.g. vertical) direction as described with reference to FIGS. 17 and 18of U.S. Pat. No. 5,410,625. In this way a single input beam can be splitinto M×N beams. The two dimensional splitting can be consider as anN-way split in a first dimension (e.g. horizontal) and as an M-way in asecond dimension (e.g. vertical).

For the case of a symmetric field fed into a rectangular waveguide thatsupports multi-mode propagation in two dimension, the M-way and N-waysplits can be given by: $\begin{matrix}{L_{{W1}_{M}}^{sym} = {\frac{p\quad W_{1}^{2}}{\lambda} + \frac{W_{1}^{2}}{M\quad\lambda}}} & \left( {4a} \right) \\{L_{{W2}_{N}}^{sym} = {\frac{q\quad W_{2}^{2}}{\lambda} + \frac{W_{2}^{2}}{N\quad\lambda}}} & \left( {4b} \right)\end{matrix}$where W₁ is the guide width, W₂ is the guide depth, p and q are integersand λ is the wavelength of propagating light. Hence, selecting pW₁ andqW₂ such that L_(W1_(M))^(sym) = L_(W2_(N))^(sym)will provide an M×N field.

For the case of an asymmetric field fed into a rectangular guide thatsupports multi-mode propagation in two dimension, the M-way and N-waysplits can be given by: $\begin{matrix}{L_{{W1}_{M}}^{ASY} = {\frac{8p\quad W_{1}^{2}}{\lambda} + \frac{4W_{1}^{2}}{M\quad\lambda}}} & \left( {5a} \right) \\{L_{{W2}_{N}}^{ASY} = {\frac{8q\quad W_{2}^{2}}{\lambda} + \frac{4W_{2}^{2}}{N\quad\lambda}}} & \left( {5b} \right)\end{matrix}$where W₁ is the guide width, W₂ is the guide depth, p and q are integersand λ is the wavelength of propagating light. Again, selecting pW₁ andqW₂ such that L_(W1_(M))^(ASY) = L_(W2_(N))^(ASY)provide an M×N field. Furthermore, feeding an asymmetric fundamentalmode in to the multi-mode waveguide region is analogous to inputting amulti-mode field.

1. A multi-mode interference device comprising a hollow core multi-modewaveguide region optically coupled to at least one hollow core inputwaveguide, wherein the internal surfaces of said hollow core waveguidescarry a reflective coating.
 2. A device as claimed in claim 1 whereinthe reflective coating comprises at least one layer of material having arefractive index less than that of the waveguide core within theoperating wavelength band.
 3. A device as claimed in claim 2 wherein atleast one of the at least one layers of material carried on the internalsurface of the hollow core waveguides is metal.
 4. A device as claimedin claim 3 wherein the metal is any one of gold, silver or copper.
 5. Adevice as claimed in claim 1 wherein the reflective coating comprisesone or more layers of dielectric material.
 6. A device as claimed inclaim 1 for operation within radiation between 1.4 μm and 1.61 μm inwavelength.
 7. A device a claimed in claim 1 wherein at least one hollowcore input waveguide is a fundamental mode waveguide.
 8. A device asclaimed in claim 1 wherein the at least one hollow core input waveguideis a multi-mode waveguide.
 9. A device as claimed in claim 1 wherein thehollow core multi-mode waveguide region has a substantially rectangularcross-section.
 10. A device according to claim 9 wherein the dimensionsof the hollow core multi-mode waveguide region are selected to providere-imaging of the optical input field carried by said at least onehollow core input waveguide.
 11. A device as claimed in claim 9 whereinopposite surfaces forming the rectangular internal cross-section of thehollow core multi-mode waveguide region have substantially equaleffective refractive indices and adjacent surfaces forming therectangular internal cross-section hollow core multi-mode waveguideregion have different effective refractive indices.
 12. A device asclaimed in claim 1 wherein the hollow core multi-mode waveguide regionhas a substantially circular cross-section and the diameter and lengthof the hollow core multi-mode waveguide region are selected to providere-imaging of the optical input field carried by said at least onehollow core input waveguide.
 13. A device as claimed in claim 1 whereinthe hollow waveguides of the MMI device are formed in semiconductormaterial.
 14. A device a claimed in claim 13 wherein the semiconductormaterial comprises Silicon.
 15. A device as claimed in claims 13 whereinthe hollow core waveguides are formed using semiconductormicro-fabrication techniques.
 16. A device according to claim 15 whereinthe semiconductor micro-fabrication technique is Deep Reactive IonEtching.
 17. An device as claimed in claims 1 wherein the hollow corewaveguides are formed in a layer of plastic or polymer.
 18. An device asclaimed in claim 1 wherein the hollow core waveguides are formed fromglass.
 19. A device as claimed in claim 1 wherein the hollow corewaveguides comprises gas.
 20. A device as claimed in claim 19 whereinthe gas is air.
 21. A device as claimed in claim 19 wherein the gas isan optical gain medium.
 22. A device as claimed in claims 1 wherein thehollow core comprises liquid.
 23. An optical amplifier comprising a1-to-N way beam splitter, a multiple element optical amplifier, and abeam recombiner connected in optical series, the optical amplifieracting on at least one of the outputs of the 1-to-N way beam splitter,wherein at least one of the 1-to-N way beam splitter and beam recombinercomprise a hollow core multi-mode interference device comprising ahollow core multi-mode waveguide region optically coupled to at leastone hollow core input waveguide, wherein the internal surfaces of saidhollow core waveguides carry a reflective coating.
 24. An opticalamplifier as claimed in claim 23 wherein the 1-to-N beam splitter andthe beam recombiner both comprise hollow core multi-mode interferencedevices.
 25. An optical amplifier as claimed in claim 23 wherein the1-to-N beam splitter comprises a solid core multi-mode interferencesplitter device.
 26. An optical amplifier as claimed in claim 23 andfurther comprising phase offset means to adjust the relative phases ofthe amplified beams prior to beam recombination in the beam recombiner.27. A resonantor comprising: a partial reflector, a splitter/recombinermeans, a multi-element optical amplifier, and a reflector, the partialreflector, splitter/recombiner means, multi-element optical amplifierand reflector being arranged such that the splitter/recombiner meanssplits a single beam into N beams where N is greater than or equal to 2,each of the N beams are amplified by the multi-element otpicalamplifier, reflected by the reflector and redirected to pass backthrough the multi-element amplifier, the N beams then being recombinedby the splitter/recombiner means to form a single beam, a portion ofthat single beam exiting the resonator through the partial reflector,wherein the splitter/recombiner means is a hollow core multi-modeinterference device comprising a hollow core multi-mode waveguide regionoptically coupled ot at least on hollow core input waveguide, whereinthe internal surfaces of said hollow core waveguides carry a reflectivecoating.